For the purpose of this specification nutating motion of a machine element relative to a fixed frame is defined as the motion of the element, an axis of which intersects with and traces out a conical surface about a stationary axis of the fixed frame. In the general case, the nutating element has a net rotational motion about its axis, relative to the fixed frame. A special case of nutating motion is one in which the nutating element has no net rotational motion.
Nutating machines may be used in a diverse range of applications including those requiring transmission of inertial loads, transmission of high compression forces, and where their characteristic conical motion is required. Inertial load transmission applications may include drive mechanisms for vibrating screens and compactors, vibrating fluid and powder mixers, and vibrating grinding mills. Nutating machines may generate large inertial forces as a consequence of the amplitude of rotation of the axis of the nutating element about a stationary axis. The surface velocities and total kinetic energy of the nutating elements are usually relatively small compared with eccentric rotating machines generating equivalent inertial forces. High compression force transmission applications may include comminution equipment similar to high pressure rolls crushers, in which material is compressed between two converging surfaces until breakage occurs.
Nutating machines have been found to be particularly suitable for application in high intensity comminution processes. In this application a chamber is driven with nutating motion to produce a centrifugal acceleration field within the chamber, which contains loose grinding media and solid particles to be ground. The motion and forces within the grinding media cause progressive breakage of the solid particles at a rate determined by the centrifugal acceleration of the nutating chamber. It is a characteristic of high intensity comminution machines that very high surface loadings occur at the bearing surfaces which constrain the chamber to perform a nutating motion. This may result in excessive power losses and wear caused by high slip velocities at the contact surfaces unless the contact surfaces are suitably proportioned. Application of the nutating machine geometry as defined herein has enabled low power losses to be achieved, resulting in a machine having high mechanical efficiency.
Nutating bearings constructed in accordance with the prior art have yielded high values of power losses and wear at the bearing surfaces. The present invention provides a simple and efficient means to overcome these limitations.
FIG. 1 illustrates an example from the prior art of support bearings currently used in nutating machinery of the type to which the present invention may be applied. The figure shows a support bearing in a nutating machine as described in Australian Patent Application number 568949. Element 101 is driven with nutating motion about stationary axis 104 as constrained by complementary bearing surface pairs 106 and 108, and 107 and 109. In this example the contact paths in complementary bearing surface pairs 106 and 108, and 107 and 109 are not constrained to have equal ratios of path lengths. As a consequence, the bearing surfaces are subjected to significant slip at the contact surfaces, with consequent excessive wear and power losses. FIG. 1 also shows additional complementary spherical bearing surfaces 110 and 111, on element 101 and frame member 105 respectively, which are in close engagement and constrain the position of element 101 by coincidence of the spherical centres of surfaces 110 and 111. An important function of bearing surfaces 110 and 111 is to transfer reaction forces directed normal to stationary axis 104 from element 101 to member 105 in any plane containing stationary axis 104. Radii 130 on the nutating element 101 of FIG. 1 are smaller than the corresponding adjacent radii on frame member 105, and hence contact and load transfer does not occur at these surfaces.